1. Field of the Invention
The present invention is related generally to road conditioning apparatus and methods for reworking or restoring road surfaces, and more particularly, to method and apparatus for mixing a liquid plasticizer agent into a layer of scarified road surface material which is to be restored.
2. Description of the Prior Art
Asphalt and other bituminous materials have long been used to form smooth roadway pavement surfaces. Over a period of time, weathering and extensive use of the roadways will render the pavement surface hard and brittle, resulting in irregularities and cracking in the pavement. Cracks due to shrinkage of the pavement may develop allowing water penetration to cause further deterioration of the roadway surface. Once started, pavement deterioration increases at an accelerated rate and, unless prompt action is taken, complete reconstruction may be required. The breakdown of pavement structure may be further accelerated by the lack of proper drainage or by the presence of corrosive foreign matter, such as chemicals or road salts. Improper construction, such as the use of too little or too much asphaltic binder, an unsatisfactory grade of aggregate or inadequate compaction may cause further pavement deterioration.
The pavement deterioration is caused in part by the effect of ultraviolet radiation from the sun which causes the bituminous material to undergo a chemical reaction which changes its physical properties, and in particular causes it to lose its resiliency. This hardening effect causes cracks to form in the road surface which may become filled with an incompressible material such as sand. A conventional repair procedure is to remove the sand from the cracks by the application of compressed air and filling the cracks with asphalt.
Experience has shown that hardening of the asphalt material occurs principally in the top inch of the pavement. Improved road working apparatus and equipment have been developed to recondition and if necessary remove this weathered, hardened layer of pavement. A conventional system utilizes heating apparatus to soften the pavement, followed by apparatus to plane the surface and remove the planed material from the roadway. In some cases an additional layer of bituminous material is spread over the reworked surface and the mixture is compressed by compacting equipment.
A basic problem in restoring hardened road surfaces by raking or scarifying has been in restoring the resiliency of the asphalt in the scarified layer. One approach has been to apply intense heat to penetrate through the hardened layer prior to scarifying in order to draw the asphaltic cement or binder to the surface and recoat the aggregate that has become barren from traffic wear. During this procedure the heat softens the mixture sufficiently to enable the scarifier to loosen and stir the material. Intense heat must be applied to obtain sufficient heat penetration through the hardened pavement surface because of the low heat conductivity of the asphalt. However, the intense heat required to draw the asphaltic cement or binder to the surface cannot be applied for a sufficiently long period since the asphalt burns at a relatively low temperature. Such asphalt burning may result in damage sufficiently severe to prevent reuse of the asphalt for resurfacing purposes.
To overcome the heating problem maintenance machines have been utilized to make multiple passes over the same section of roadway in order to heat and work the pavement to a sufficient depth without burning the asphalt. However, this procedure is inefficient and time-consuming.
One approach to restoring the resiliency of the hardened layer is to alternately heat the paved surface and allow the surface to partially cool until the surface has been heated and softened to a predetermined depth while maintaining the temperature of the surface below a predetermined limit. In combination with this procedure, after the softened layer has been scarified, a plasticizing agent comprising petroleum products is sprayed upon the exposed top surface of the scarified layer of pavement to rejuvenate the hardened asphalt. The distributor is typically a tank carried on a truck which follows the scarifier and thus delays the application of the roller which compacts the scarified pavement. Experience has demonstrated that the plasticizing agent when sprayed upon the scarified pavement in that manner will not effectively penetrate the scarified layer. Therefore, only the exposed surface of the scarified material is coated with the plasticizing agent. During the period that the plasticizing agent is being sprayed upon the exposed surface of the scarified material, the temperature of the pavement generally decreases. If the temperature decreases sufficiently, the viscosity of the asphalt increases beyond the point at which adequate compaction is possible. Therefore there is needed apparatus for use with a road working machine for mixing the rejuvenating agent uniformly throughout the scarified layer so that surface compaction can be carried out with minimum delay after heating has occurred.